1. Field of the Invention
The present invention relates to a wavelength interleaver, and more particularly, to a wavelength interleaver with small size and reducing the cost of the signal transmission system.
2. Description of the Related Art
As technologies change with each passing day, population increasing and internet users increasing, the internet provider provides more bandwidths for transmitting the increasing audio and video signals according to the increasing requirement of communication transmission. Technologies of wavelength division multiplexing (WDM) and dense wavelength division multiplexing (DWDM) are capable of expanding the total transmission capacity of the existing optical fiber framework. Dense wavelength division multiplexing (DWDM) could transmit optical wavelengths with various signals through a single optical fiber. Optical filtering membranes are used for multiplexing/de-multiplexing and adding/dropping the optical wavelengths. However, the channel spacing of the optical filtering membranes is hardly to reduce and the optical filtering membranes may be aged along with time in a high power condition. Thus, decreasing spacings between adjacent channels of the transmission system is a better way for increasing the total transmission capacity.
FIG. 1 is a block diagram of a traditional 1×2 wavelength interleaver. The 1×2 wavelength interleaver 10 separates optical input signals 20 into odd optical signals 201 and even optical signals 202. The spacing are changed from f to 2f. For the system framework of existing optical filtering membranes speaking, frequency spacings between adjacent channels are 200 GHz. If the frequency spacings between adjacent channels are lowered to 100 GHz, the dense signals are, then, converted to wide signals. Assuming the network framework of the optical filtering membranes is invariable, the channel is, therefore, doubled for doubling the total transmission capacity. Moreover, the wavelength interleaver 10 could be a 1×4 wavelength interleaver, a 4×8 wavelength interleaver, a 1×8 wavelength interleaver or a M×N wavelength interleaver (both M and N are integers). Different types of the wavelength interleavers have different applications. Users can choose a suitable type wavelength interleaver for use according to the requirement.
Generally speaking, micro-optics and planar lightwave circuit (PLC) are the two ways for interleaving wavelengths. A disadvantage of the former one is the great size and the size may be greater after multi-stage optimization. The disadvantage of planar lightwave circuit (PLC) is the small size and the size is still small after multi-stage optimization. Furthermore, the cost of crystal used in micro-optics is higher than the cost of the planar lightwave circuit (PLC) manufacturing through semiconductor manufacture processes.
FIG. 2(a) is a hint diagram of a traditional 1×2 wavelength interleaver accomplished by planar lightwave circuit (PLC). The 1×2 wavelength interleaver is a Mach-Zehnder wavelength interleaver. FIG. 2(b) is a transmission rate waveform diagram of the Mach-Zehnder wavelength interleaver. As shown in FIG. 2(b), the Mach-Zehnder wavelength interleaver is not practical due to the problems of great crosstalk between the first output and the second output and dispersion.
FIG. 3 is a hint diagram with an adding ring filter in the wavelength interleaver shown in FIG. 2(a). That is, the ring filter provides a non-linear phase modulation with less attenuation or without attenuation. The two 3 dB couplers attenuate the input signals and provide a linear phase shifting to the input signals. There is a sufficient band to separate the channels (optical wave channels) of each input signal by the combination of the two 3 dB couplers for attenuating input signals and the ring filter for non-linear phase modulation. Therefore, problems of crosstalk and dispersion are solved. However, the band is narrowed for decreasing the channel bandwidth after transmitting input signals several times. Accordingly, the whole transmission system should be developed with higher precision for avoiding errors causing by the narrowed bandwidth of optical input signals with a single wavelength. Thus, the cost and size of the whole transmission system is great increasing.
It is desirable, therefore, to provide a wavelength interleaver with small size and reducing the cost of the transmission system.